Radiation detectors are used for detection of incoming radiation such as X-rays, gamma photons and charged/uncharged particles, in a wide range of different applications. For detection of photons of various energies, the incoming photons are converted to electrons through electromagnetic interactions, including the photoelectric effect, pair production and the Compton effect. The emitted electrons are normally multiplied by a multiplication structure to produce a multiplicity of secondary electrons and/or photons, which in turn may be detected by a suitable sensor device.
For low-energy photons, gas conversion is very successful with almost 100% efficiency combined with high spatial resolution. However, as the photon energy increases, gas conversion becomes less attractive since the photoelectric capture cross section falls rapidly and the photoelectron range increases. This results in degraded efficiency, as well as heavily deteriorated spatial resolution due to the extended tracks of the long-range electrons.
For higher photon energies, a successful approach is to provide gaseous radiation detectors with a solid converter to increase the probability of electromagnetic interaction with the incoming radiation. Such a converter is needed in order to increase the efficiency since higher energy photons are much more penetrating and would otherwise pass the detector undetected. Compared to non-gaseous detectors, there are several advantages of such an approach, including improved efficiency, low price and larger sensitive area. These advantages have stimulated many researchers to develop gaseous detectors combined with solid converters.
Probably one of the first attempts to combine gaseous detectors and solid converters was made by Jeavon et al, as described in the article The High-Density Multiwire Drift Chamber, Nuclear Instruments and Methods, 124, 1975, pp. 491-503. Jeavon and his colleagues suggested to use a stack of perforated solid gamma converters combined with a multiwire proportional chamber. As illustrated in FIG. 1, a large number of metal plates (copper) 12 are interleaved with Mylar sheets 14. The complete stack is perforated to form drift holes into which photoelectrons emitted from the copper plates 12 can be released. Secondary electrons resulting from gas ionization in the drift holes are extracted by an electric drift field and detected by a multiwire proportional chamber 16.
In the article High resistance Lead Glass Tubing for Rich Counters and for Electromagnetic Calorimeters in Nuclear Instruments and Methods in Physics Research, A257, 1987, pp. 609-613, Del Guerra et al. suggested to use an array of lead glass tubing combined with a multiwire proportional chamber. The lead glass tubing acts as a combined gamma converter and electron drift structure, with the possibility of gas multiplication inside the lead glass capillaries.
However, both of these developments have not received any wide spread use due to the fact that the efficiency of the converters is still relatively low (a few %). The low efficiency is associated with the fact that electrons created by gamma radiation inside the converters have a very short mean free path (normally less than a fraction of a mm). As a result, only electrons created in the converters near the inner walls to the holes or capillaries can penetrate freely into the gas volume.
There have also been attempts to develop gaseous detectors with X-ray converters. As described in U.S. Pat. No. 5,192,861 issued to Breskin et al. on Mar. 9, 1993, a thin flat cesium iodide (CsI) layer was used as a converter. However, the efficiency of the CsI converter layer is also very low for the same reason as mentioned above. In addition, the useful surface of the converter is actually rather small due to the acute angle necessary to obtain a reasonable efficiency. As a result, this type of detector has not gained any practical application.
The approach of combining gaseous detectors with solid converters has recently gained some new interest in the International Patent Application WO 01/59478 by Brahme et al. published on Aug. 16, 2001. The diagnostic and therapeutic detector system proposed by Brahme et al. is intended for imaging with both X-ray and gamma photons, and is based on a stack of well aligned, alternating perforated gamma converter layers 22 and gas electron multiplier layers (GEMs) 24, as schematically illustrated in FIG. 2. The top layer in the alternating stack is preferably a GEM 24 for multiplication of electrons generated in the top gas volume. In the gas volume between the inlet window and the top GEM, diagnostic X-rays will interact with the gas and emit electrons, which are collected and amplified by the GEM structures 24 in the stack. For photons in the radiation therapy beam, the top gas volume will be more or less transparent, and such higher energy photons penetrates into the stack and converts into electrons in the different converter layers 22. High-energy photons of relatively lower energy will predominantly convert in the top converter layers, while photons of relatively higher energy will dominate in the bottom layers. At the bottom, a sensor device 26 is arranged for collection of the electrons or photons resulting from the alternating converter and multiplier stack.
Although the detector system of Brahme et al. constitutes a significant improvement, especially for gamma radiation, the X-ray efficiency and position resolution is still relatively poor due to the large mean free path of the converted electrons in the gas.
Various other attempts to improve the X-ray efficiency in gaseous radiation detectors include using inclined capillaries, as described in U.S. Pat. No. 6,333,506 issued to Francke et al. on Dec. 25, 2001.